Coupling systems for interconnecting lines, particularly fluid conveying lines, must provide reliable service even in adverse operating conditions. For example, the couplings for fluid lines associated with the hydraulic, fuel, or other systems of aircraft are subjected to vibration, temperature extremes, axial loading and a variety of other difficult operating conditions. Reliably insuring such couplings are properly connected is essential to safe operation as failure of such couplings or their improper installation could lead to catastrophic results.
Shown in FIG. 1 is a typical fluid coupling 8 comprising a coupling element 9 having a ferrule 10 with a frustoconical seat 11, which acts as a sealing surface, and a second coupling element 12 comprising a ferrule with a complementary sealing surface, depicted as a ball nose surface 13. Coupling elements 9 and 12 are joined together by a coupling nut 14 on the first coupling element 9 being threaded onto threads of the second coupling element 12 and tightened, transmitting axial forces from the coupling nut 14 through a load bearing surface 15 to the complementary sealing surfaces 11 and 13. This axial compression forces these complementary sealing surfaces together to achieve a pre-load of one surface against the other such that the surfaces will remain fluid tight through the operational environment of the fluid coupling. Thread torque is conventionally used as a measure of whether an appropriate compressive pre-load of the sealing surfaces to achieve the fluid-tight seal has been reached, and also as a measure that the threaded surfaces are sufficiently engaged to prevent loosening. Loosening of an engaged and torqued coupling is also accomplished by the use of lockwire and other apparatus, such as disclosed in co-pending application Ser. No. 07/815,998, now U.S. Pat. No. 5,188,398, Redundantly Locked Fluid Coupling, to Parimore, et al. incorporated herein by reference. While it is important for the coupling elements to be tightened sufficiently to form a fluid-tight seal, it is also essential that the elements not be overtightened. Overtightening of a coupling can lead to not only weakening and failure of the threads of either coupling element, but also to plastic deformation of either the seat or mating sealing surface, or both.
The problem of applying the proper amount of axial pre-load to mating coupling elements has been addressed by providing assembly personnel with torque wrenches designed for a given coupling. However, this solution often produces less than desired results in practice. It is not unusual for a mechanic or other assembler to employ the wrench or other tool at hand, rather than search for the torque wrench designed for a given coupling. This results in an unreliable estimate of the amount of torque and the potential for either overtightening and failure of the coupling or undertightening and a potential loosening and leakage of the coupling elements. Even if the appropriate torque wrench is employed, proper seating pre-load to assure a leak-free joint is not guaranteed. A torque wrench measures the amount of torque applied to a coupling. The torque wrench does not measure the amount of axial seating pre-load a joint receives. It is this axial seating load that assures a leak-free joint. Torque vs. axial seating pre-load is a function of many variables. With today's couplings it is possible, with damaged threads for example, to torque a fitting to the proper torque range and still not achieve the proper seating pre-load.
In addition to torque not being a reliable measure of axial seating pre-load, aircraft threaded connections are subjected to vibration and other operating conditions which make such connections susceptible to loosening after a period of time, thus no longer providing a fluid-tight connection. In many coupling designs, such loosening of the coupling elements cannot be visually detected and maintenance personnel often check on the integrity of the coupling connection by taking the coupling apart and then retightening it with the appropriate torque wrench or other means. This can increase the time and cost of maintenance particularly in instances where the coupling elements have not in fact become loosened and could have been left intact.